From af88cec22df5ad39e7670a38d4ba35c7fc21769e Mon Sep 17 00:00:00 2001
From: Steve Plimpton <sjplimp@gmail.com>
Date: Fri, 5 Apr 2024 15:12:46 -0600
Subject: [PATCH] relax error check on fixes that produce variable size
 vectors/arrays

---
 src/thermo.cpp   | 20 ++++++++++++--------
 src/variable.cpp | 16 ++++++++++++----
 2 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/src/thermo.cpp b/src/thermo.cpp
index 5ef5eb59b8..128f8573cf 100644
--- a/src/thermo.cpp
+++ b/src/thermo.cpp
@@ -1515,15 +1515,16 @@ void Thermo::compute_fix()
   int m = field2index[ifield];
   Fix *fix = fixes[m];
 
-  // check for out-of-range access if vector/array is variable length
-
   if (argindex1[ifield] == 0) {
     dvalue = fix->compute_scalar();
     if (normflag && fix->extscalar) dvalue /= natoms;
   } else if (argindex2[ifield] == 0) {
-    if (fix->size_vector_variable && argindex1[ifield] > fix->size_vector)
-      error->all(FLERR, "Thermo fix vector is accessed out-of-range");
-    dvalue = fix->compute_vector(argindex1[ifield] - 1);
+
+    // if index exceeds variable vector length, use a zero value
+    // this can be useful if vector length is not known a priori
+
+    if (fix->size_vector_variable && argindex1[ifield] > fix->size_vector) dvalue = 0.0;
+    else dvalue = fix->compute_vector(argindex1[ifield] - 1);
     if (normflag) {
       if (fix->extvector == 0)
         return;
@@ -1533,9 +1534,12 @@ void Thermo::compute_fix()
         dvalue /= natoms;
     }
   } else {
-    if (fix->size_array_rows_variable && argindex1[ifield] > fix->size_array_rows)
-      error->all(FLERR, "Thermo fix array is accessed out-of-range");
-    dvalue = fix->compute_array(argindex1[ifield] - 1, argindex2[ifield] - 1);
+
+    // if index exceeds variable array rows, use a zero value
+    // this can be useful if array size is not known a priori
+
+    if (fix->size_array_rows_variable && argindex1[ifield] > fix->size_array_rows) dvalue = 0.0;
+    else dvalue = fix->compute_array(argindex1[ifield] - 1, argindex2[ifield] - 1);
     if (normflag && fix->extarray) dvalue /= natoms;
   }
 }
diff --git a/src/variable.cpp b/src/variable.cpp
index c99e4a4761..896b193313 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -1807,12 +1807,16 @@ double Variable::evaluate(char *str, Tree **tree, int ivar)
 
             if (!fix->vector_flag)
               print_var_error(FLERR,"Mismatched fix in variable formula",ivar);
-            if (index1 > fix->size_vector)
+            if (index1 > fix->size_vector && fix->size_vector_variable == 0)
               print_var_error(FLERR,"Variable formula fix vector is accessed out-of-range",ivar,0);
             if (update->whichflag > 0 && update->ntimestep % fix->global_freq)
               print_var_error(FLERR,"Fix in variable not computed at a compatible time",ivar);
 
-            value1 = fix->compute_vector(index1-1);
+            // if index exceeds variable vector length, use a zero value
+            // this can be useful if vector length is not known a priori
+
+            if (fix->size_vector_variable && index1 > fix->size_vector) value1 = 0.0;
+            else value1 = fix->compute_vector(index1-1);
             argstack[nargstack++] = value1;
 
           // f_ID[i][j] = scalar from global array
@@ -1821,14 +1825,18 @@ double Variable::evaluate(char *str, Tree **tree, int ivar)
 
             if (!fix->array_flag)
               print_var_error(FLERR,"Mismatched fix in variable formula",ivar);
-            if (index1 > fix->size_array_rows)
+            if (index1 > fix->size_array_rows && fix->size_array_rows_variable == 0)
               print_var_error(FLERR,"Variable formula fix array is accessed out-of-range",ivar,0);
             if (index2 > fix->size_array_cols)
               print_var_error(FLERR,"Variable formula fix array is accessed out-of-range",ivar,0);
             if (update->whichflag > 0 && update->ntimestep % fix->global_freq)
               print_var_error(FLERR,"Fix in variable not computed at a compatible time",ivar);
 
-            value1 = fix->compute_array(index1-1,index2-1);
+            // if index exceeds variable array rows, use a zero value
+            // this can be useful if array size is not known a priori
+
+            if (fix->size_array_rows_variable && index1 > fix->size_array_rows) value1 = 0.0;
+            else value1 = fix->compute_array(index1-1,index2-1);
             argstack[nargstack++] = value1;
 
           // F_ID[i] = scalar element of per-atom vector, note uppercase "F"